TORONTO — It was the day before shooting began on "The Fifth Estate" when Benedict Cumberbatch, as he was trying on blond wigs for the last time, heard back from Julian Assange, the man he was to play.

Through friends of the WikiLeaks creator, Cumberbatch had reached out to Assange. The letter (which WikiLeaks has since posted online) starts out cordially, with Assange complementing Cumberbatch's work. But he then levels a warning: "I believe you are a good person, but I do not believe that this film is a good film."

Assange went on to say, at some length, why the movie, based on two books Assange disputes (Daniel Domscheit-Berg's "Inside WikiLeaks: My Time With Julian Assange and the World's Most Dangerous Website" and "WikiLeaks: Inside Julian Assanges War on Secrecy," by British journalists David Leigh and Luke Harding), would do harm to him, WikiLeaks and whistleblowers around the world.

"It winded me quite a bit," Cumberbatch said in an interview shortly after "The Fifth Estate" premiered at the Toronto Film Festival last month. "He was very polite. He wasn't aggressive. ... But there was a certain amount of naivety about what he thought a film's impact could be."

Playing Assange, one of the world's most debated and recognizable figures, was always going to be a challenge for Cumberbatch, and it would be particularly so with his very subject objecting to the enterprise. Cumberbatch hoped his portrait would be balanced, showing Assange as controlling and reckless, but also "fiercely intelligent, principled and rather brilliant."

"Frankly, the perception of him before this film comes out is already a fractured thing," Cumberbatch says. "The kind of tabloid heading of it is 'Weirdo, white-haired guy wanted for rape who's hiding behind Harrod's in an embassy' — nothing to do with the achievements we show in the film. All of that is celebrated."

"The Fifth Estate," which Disney will release Oct. 18, is just the first entry in a season of Cumberbatch, the British actor most famed for his running TV series "Sherlock." This fall, he also appears in "August: Osage County," ''12 Years a Slave" and "The Hobbit: The Desolation of Smaug."

Cumberbatch is the latest British sensation in Hollywood — an actor of scrupulous intelligence and a fragile, shape-shifting presence as unique as his name.

"At the moment, I'm just enjoying the momentum," he says.

But Cumberbatch hasn't been enthralled with every aspect of his growing fame. When hounded by paparazzi earlier this year, he, hooded and wearing sunglasses, held a sign over his face reading: "Go photograph Egypt and show the world something important." The actor sympathizes with Assange, whom he believes has been more scrutinized personally than discussed for his work: "I don't want everyone to know everything about me," he says.

Cumberbatch is known for his energy and boundless loquaciousness, and will go on for many uninterrupted minutes turning over all sides of a question.

What most comes across talking to Cumberbatch, the only child of actors Wanda Ventham and Timothy Carlton, is his thoroughness. To craft Assange, he poured over YouTube footage of him, talked with Assange colleagues and worked with a dialect coach to specify Assange's droll Australian accent.

"It's an excuse for one of the gifts of our job, which is further education," he says. "Within that, I always get embroiled in the detail. ... I believe in the intelligence of audiences and I also believe they deserve hard work."

"With an actor like that, it's just making sure that all of the levels of complexity are communicated," Condon says. "That's his great gift. There are moments when he can express something that's a moment of great intelligence combined with some cruelty and also vulnerability."

Often, Cumberbatch found himself flip-flopping on Assange, judging him cantankerous in an interview, then re-watching and seeing the unfair bias of the interviewer.

"It was very difficult to compartmentalize the work in building toward him," says Cumberbatch. "Every single moment had a huge deal of potential interpretation..."

He's clearly still enraptured by the contradiction of Assange, whom he analyzes with nearly Sherlock-like precision. Yet Cumberbatch says he doesn't feel similar to the fast-thinking detective, whom he says is "a real effort to work up to."

Still, it can be hard to slow Cumberbatch once he gets going. He recalls an interview where he gave a short answer that led to a quiet pause of confusion before he explained that he had indeed stopped talking.